Several surveillance systems are known, but suffer the drawbacks of being rather clumsy to use mainly due to electrical connections being thin and sensitive to dirt, oil, humidity, and so forth. As for example so called strain gauges are concerned the electrical conduits normally have to be soldered to the object, which makes prefabrication impossible and the manufacturing costs get important.
Most of the abovementioned is also true for so called piezoresistive/ceramic gauges which further are very brittle and thus very sensitive to e.g. cracks.
Alternatively so called ultrasonic techniques can be used. This is however both expensive and time consuming partly because the ultrasonic device has to be mounted separately for every object, thus the need for a more continuous survey cannot be met by these techniques.
In DE-C-38 19 083 a magnetoelastic sensor system is disclosed which comprises sensor means comprising a magnetoelastic material sensitive to the preloads to which a bolt is exposed, said system further comprising a coil system, surrounding the magnetoelastic material.
There is however no indication of a complete surveillance system.
The system is furthermore clumsy to handle and sensitive to stray fields. In EP-A-0 422 702 a magnetoelastic torque transducer is shown. Also here no surveillance system is discussed and the magnetoelastic material is excited with a frequency of 1-100 kHz, 20 kHz being the preferred frequency in order to avoid whistling. Also this sensor is sensitive to stray fields which makes it unpractical.
It is well known to measure torque applied to a rotary or fixed shaft by use of magnetoelastic materials which form a part of, or are arranged on the shaft, since it is known that the permeability of magnetic materials changes due to applied stress. The permeabilities of the magnetoelastic materials change due to the fact that a magnetoelastic material changes its magnetic properties when it is subject to tensile and compressive stresses, respectively. Therefore, the torque and/or axial stresses to which an object is exposed must either be transmitted to tensile and compressive stresses of the magnetoelastic material which is fastened onto the object, or the object itself (or a part of it) comprises a magnetoelastic material.